1. Field of the Invention
This invention relates to a prosthetic intervertebral disc nucleus. More particularly it relates to an artificial disc nucleus made of a hydrogel material.
The intervertebral disc is a complex joint anatomically and functionally. It is composed of three component structures: the nucleus pulposus, the annulus fibrosus and the vertebral end-plates. The biomedical composition and anatomical arrangements within these component structures are related to the biomechanical function of the disc.
The nucleus pulposus occupies 25-40% of the total disc cross-sectional area. It is composed mainly of mucoid material containing mainly proteoglycans with a small amount of collagen. The proteoglycans consist of a protein core with chains of negatively charged keratin sulphate and chondroitin sulphate attached thereto. Due to these constituents, the nucleus pulposus is a "loose or amorphous hydrogel" which has the capacity to bind water and usually contains 70-90% water by weight. Although the nucleus plays an important role in the biomechanical function of the disc, the mechanical properties of the disc are not well known, largely because of the loose hydrogel nature of the nucleus.
Because the nucleus is surrounded by the annulus fibrosus and vertebral end-plates, and the negatively charged sulphate groups are immobilized due to the attachment of these groups to the polymer matrix, this causes the matrix to have a higher concentration of ions than its surroundings. This higher concentration results in a higher osmotic pressure ranging between 0.1-0.3 MPa. As a result, the high fixed charge density of the proteoglycan leads the matrix to exert an osmotic swelling pressure which can support an applied load in much the same way as air pressure in a tire supports the weight of a car.
It is the osmotic swelling pressure and hydrophilicity of the nucleus matrix that offers the nucleus the capability of imbibing fluid until it is balanced with the resistance stresses coming internally from the tensile forces of the collagen network and externally from the loads that are applied by muscle and ligament tension. The swelling pressure (Ps) of the nucleus is dependent on the concentration of proteoglycan, i.e. the higher proteoglycan concentration, the higher swelling pressure of the nucleus, and vise versa. This external pressure changes with posture. When the human body is supine, the compressure load on the third lumbar disc is 300 newtons (N), which rises to 700 N when upright stance is assumed and to 1200 N when bending forward by only 20.degree.. When the external pressure (Pa) increases, it breaks the previous balance of Ps=Pa. To reach the new balance, the swelling pressure (Ps) has to increase. This increase is achieved by increasing the proteoglycan concentration in the nucleus which is in turn achieved by reducing the fluid in the nucleus. That is why discs will lose about 10% of their height as a result of creep during the daytime. When the external load is released (Ps&gt;Pa), the nucleus will imbibe fluid from its surroundings in order to reach the new equilibrium. It is this property of the nucleus that is mainly responsible for the compressive properties of the disc.
The annulus fibrosus forms the outer limiting boundary of the disc. It is composed of highly structured collagen fibers embedded in amorphous base substance also composed of water and proteoglycans, which is lower in content in the annulus than in the nucleus. The collagen fibers of the annulus are arranged in concentric laminated bands (8-12 layers thick) with a thicker anterior wall and thinner posterior wall. In each lamella, the fibers are parallel and attached to the superior and inferior vertebral bodies at roughly a 30.degree. angle from the horizontal plane of the disc in both directions. This design particularly resists twisting, as half of the angulated fibers will tighten as the vertebrae rotate relative to each other in either direction.
The composition of the annulus fibrosus along the radial axis is not uniform. There is a steady increase in the proportion of collagen from the inner to the outer annulus. This difference in composition may reflect the need of the inner and outer regions of the annulus to blend into very different tissues while maintaining the strength of the structure. Only the inner lamellae are anchored to the end-plates forming an enclosed vessel for the nucleus. The collagen network of the annulus restrains the tendency of the nucleus gel to absorb water from surrounding tissues and swell. Thus, the collagen fibers in the annulus are always in tension, and the nucleus gel is always in compression.
The two vertebral end-plates are composed of hyaline cartilage, which is a clear "glassy" tissue, and separates the disc from the adjacent vertebral bodies. This layer acts as a transitional zone between the hard, bony vertebral bodies and the soft disc. Because the intervertebral disc is avascular, most nutrients that the disc needs for metabolism are transported to the disc by diffusion through the end-plate area.
The intervertebral joint exhibits both elastic and viscous behavior. Hence, during the application of a load to the disc there will be an immediate "distortion" or "deformation" of the disc, often referred to as "instantaneous deformation". It has been reported that the major pathway by which water is lost from the disc during compression is through the cartilage end-plates. Because the permeability of the end-plates is in the range (0.20 -0.85).times.10.sup.-17 m.sup.4 N.sup.-1 sec.sup.-1, it is reasonable to assume that under loading, the initial volume of the disc is constant while the load is applied. Because the natural nucleus of the disc is in the form of loose hydrogel which can be deformed easily, the extent of deformation of the disc is largely dependent on the extensibility of the annulus. It is generally believed that the hydrostatic behavior of the nucleus pulposus plays an important role in the normal static and dynamic load-sharing capability of the disc and the restoring force of stretched fibers of the annulus balances the effects of nucleus swelling pressure. Without the constraint from the annulus, nucleus annular bulging increases considerably. If the load is maintained at a constant level, a gradual change in joint height will occur as a function of time which is commonly referred to as "creep". Eventually, the creep will stabilize and the joint is said to be in "equilibrium". When the load is removed the joint will gradually "recover" to its original height before loading (the creep and relax rate depends on the amount of load applied, the permeability of the end-plates and the water binding capability of the nucleus hydrogel). The creep and relax is an essential process to pumping the fluid in and out of the disc.
Degeneration of the intervertebral disc is believed to be a common cause of final pathology and of back pain. As the intervertebral disc ages, it undergoes degeneration. The changes that occur are such that in many respects the composition of the nucleus seems to approach that of the inner annulus fibrosus. Intervertebral disc degeneration is, at least in part, the consequence of the composition change of the nucleus. It has been found that both the molecular weight and the content of proteoglycans from the nucleus decreases with age, especially in degenerated discs, and the ratio of keratin sulphate to chondroitin sulphate in the nucleus increases. This increase in the ratio of keratin sulphate to chondroitin sulphates and decrease in proteoglycan content decreases the fixed charge density of the nucleus from 0.28 meq/ml to about 0.18-0.20 meq/ml. These changes cause the nucleus to lose its water binding capability and its swelling pressure. As a result, the nucleus becomes less hydrated, and its water content drops from over 85% in preadolescence to about 70-75% in middle age. The glycosaminoglycan content of prolapsed discs has been found to be lower, and the collagen content higher than that of normal discs of a comparable age. Discs L-4- L-5 and L-5 - S-1 are usually the most degenerated discs.
It is known that although the nucleus only has about one third of the total disc area, it takes about 70% of the total loading in a normal disc. It has been found that the load in the nucleus of moderately degenerated discs is 30% lower than in comparable normal discs. However, the vertical load on the annulus fibrosus increases by 100% in the degenerated discs. This load change is primarily caused by the structural changes of the disc as discussed above. The excess load on the annulus of the degenerated discs would cause narrowing of the disc spaces and excessive movement of the entire spinal segments. The flexibility would produce excessive movement of the collagenous fibers that, in turn, would injure the fiber attachments and cause delamination of the well organized fibers of the annulus ring. The delaminated annulus can be further weakened by stress on the annulus and in severe cases this stress will cause tearing of the annulus. This whole process is very similar to driving on a flat tire, where the reinforcement layer will eventually delaminate. Because the thickness of the annulus is not uniform, with the posterior being thinner than the anterior, the delamination and the lesion usually occur in the posterior area first.
The spinal disc may also be displaced or damaged due to trauma or a disease process. In this case and in the case of disc degeneration, the nucleus pulposus may herniate and/or protrude into the vertebral canal or intervertebral foramen, in which case it is known as a herniated or "slipped" disc. This disc may in turn press upon the spinal nerve that exits the vertebral canal through the partially obstructed foramen, causing pain or paralysis in the area of its distribution. The most frequent site of occurrence of a herniated disc is in the lower lumbar region. A disc herniation in this area often involves the inferior extremities by compressing the sciatic nerve.
There are basically three types of treatment currently being used for treating low back pain caused by injured or degenerated discs: conservative care, laminectomy and fusion. Each of these treatments has its advantages and limitations. The vast majority of patients with low back pain, especially those with first time episodes of low back pain, will get better with conservative care treatment. However, it is not necessarily true that conservative care is the most efficient and economical way to solve the low back pain problem.
Laminectomy usually gives excellent short term results in relieving the clinical symptoms by removing the herniated disc material (usually the nucleus) which is causing the low back pain either by compressing the spinal nerve or by chemical irritation. Clearly a laminectomy is not desirable from a biomechanical point of view. In the healthy disc, the nucleus takes the most compressional load and in the degenerated disc this load has been distributed more onto the annulus ring, which, as described above, causes tearing and delamination. The removal of the nucleus in a laminectomy actually causes the compressive load to be distributed further on the annulus ring, which would narrow the disc spaces. It has been reported that a long-term disc height decrease might be expected to cause irreversible osteo-arthritic-like changes in the adjacent facet joint. That is why laminectomy has poor long term results and high incidence of reherniation.
Fusion generally does a good job in eliminating symptoms and stabilizing the joint. However, because the motion of the fused segment is restricted, it increases the range of motion of the adjoining vertebral discs, possibly enhancing their degenerative processes.
Because of these disadvantages, it is desirable to develop a prosthetic joint device which not only is able to replace the injured or degenerated intervertebral disc, but also can mimic the physiological and the biomechanical function of the replaced disc. Such a device would restore the function of the disc and prevent further degeneration of the surrounding tissue.
2. Description of the Prior Art
Various artificial discs are well known. U.S. Pat. No. 3,867,728 to Stubstad et al, which issued on Feb. 25, 1975, relates to a device which replaces the entire disc. This device is made by laminating vertical, horizontal or axial sheets of elastic polymer. U.S. Pat. No. 3,875,595 to Froning, dated Apr. 8, 1975, relates to a collapsible plastic bladder-like prosthesis of nucleus pulposus. Another U.S. patent relates to a prosthesis utilizing metal springs and cups (Patil, U.S. Pat. No. 4,309,777). A spinal implant comprising a rigid solid body having a porous coating on part of the surface is shown in Kenna's U.S. Pat. No. 4,714,469. An intervertebral disc prosthesis of a pair of rigid plugs to replace the disc is shown in Kuntz, U.S. Pat. No. 4,349,921. Ray et al, U.S. Pat. Nos. 4,772,287 and 4,904,260, use a pair of cylindrical prosthetic intervertebral disc capsules with or without therapeutical agents. U.S. Pat. No. 4,911,718 relates to an elastomeric disc spacer comprising three different parts; nucleus, annulus and end-plates, of different materials. At the present time, none of these concepts has become a product in the spinal care market.
The main reason for the difficulty in implementing these concepts is that except for the concepts of Froning's, Kuntz's and Ray's, these prostheses call for replacing the entire natural disc, which involves numerous surgical difficulties. Secondly, the intervertebral disc is a complex joint anatomically and functionally and it is composed of three component structures, each of which has its own unique structural characteristics. To design and fabricate such a complicated prosthesis from acceptable materials which will mimic the function of the natural disc is very difficult. A problem also exists in finding a way to prevent the prosthesis from dislodging. Thirdly, even for prostheses which are only intended for replacing the nucleus, a major obstacle is finding a material which is in character similar to the natural nucleus and also is able to restore the natural function of the nucleus. Neither silicone elastomers nor thermoplastic polymers are ideal for the prosthetic nucleus due to their significant inherent characteristic differences from the natural nucleus.
This problem is not solved by Kuntz, which involves using elastic rubber plugs, and Froning and Ray, which use bladders filled with a fluid or plastic or thixotropic gel. In both the latter cases, liquid was used to fill the bladder so that the bladder membrane had to be completely sealed to prevent fluid leakage. Clearly, the prior devices would not completely restore the function of the nucleus which allows the fluid to diffuse in and out during cyclic loading to allow body fluid diffusion which provides the nutrients the disc needs.
This invention relates to a new prosthetic lumbar disc nucleus which is made from synthetic hydrogels. Hydrogels have been used in biomedical applications in various areas such as contact lenses. Among the advantages of hydrogels are that they are more biocompatible than other hydrophobic elastomers and metals. This biocompatibility is largely due to the unique characteristics of hydrogels in that they are soft and hydrated like the surrounding tissues and have relatively low friction with respect to the surrounding tissues. The biocompatibility of hydrogels results in a prosthetic nucleus more easily tolerated in the body.
An additional advantage is that some hydrogels have good mechanical strength which permits them to withstand the load on the disc and restore the normal space between the vertebral body. The prosthetic nucleus of the present invention has high mechanical strength and is able to withstand the body load and assist in the healing of the defective annulus.
Another advantage of the present invention is that many hydrogels have excellent visco-elastic properties and shape memory. Unlike other elastomeric polymers, hydrogels contain a large portion of water which acts as a plasticizer. Part of the water in the hydrogel is available as free water, which has more freedom to leave the hydrogel when the hydrogel is partially dehydrated or under mechanical pressure. This characteristic of the hydrogels enables them to creep in the same way as the natural nucleus under compression and to withstand cyclic loading for long periods without any significant degradation and without losing their elasticity. This is because water in the hydrogel behaves like a cushion which makes the network of the hydrogel less stretched.
In addition, hydrogels are permeable to water and water-soluble substances, such as nutrients, metabolites and the like. It is known that body fluid diffusions under cyclic loading is the major source of nutrients to the disc and if the route of this nutrient diffusion is blocked, it will cause further deterioration of the disc.
The hydrogels used in the disc of the present invention, as with many hydrogels, can be dehydrated and then hydrated again without changing the properties of the hydrogel. When the hydrogel is dehydrated, its volume will decrease, which makes it possible to implant the prosthetic nucleus in the dehydrated or unhydrated state. The implanted prosthetic nucleus will then swell slowly in the body. This feature makes it possible to implant the device laterally during surgery, thereby reducing the complexity and risk of intraspinal surgery traditionally used. The danger of perforation of the nerve, dural sac, arteries and other organs is reduced. The incision area on the annulus also can be reduced, thereby helping the healing of the annulus and preventing the reherniation of the disc. Hydrogels have also been used in drug delivery due to their capability for a controllable release of the drug. Different therapeutic agents, such as different growth factors, long term analgesics and antiinflammatory agents can be attached to the prosthetic nucleus and be released in a controllable rate after implantation.
Furthermore, dimensional integrity is maintained with hydrogels having a water content of up to 90%. This dimensional integrity, if properly designed, distributes the load to a larger area on the annulus ring and prevents the prosthetic nucleus from bulging and herniating.